首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3479篇
  免费   13篇
  国内免费   5篇
航空   1550篇
航天技术   1253篇
综合类   181篇
航天   513篇
  2021年   21篇
  2019年   23篇
  2018年   63篇
  2017年   36篇
  2016年   39篇
  2015年   21篇
  2014年   77篇
  2013年   92篇
  2012年   79篇
  2011年   128篇
  2010年   75篇
  2009年   142篇
  2008年   176篇
  2007年   95篇
  2006年   87篇
  2005年   91篇
  2004年   97篇
  2003年   100篇
  2002年   164篇
  2001年   165篇
  2000年   53篇
  1999年   86篇
  1998年   101篇
  1997年   80篇
  1996年   102篇
  1995年   120篇
  1994年   85篇
  1993年   51篇
  1992年   78篇
  1991年   32篇
  1990年   27篇
  1989年   67篇
  1988年   26篇
  1987年   25篇
  1986年   29篇
  1985年   113篇
  1984年   99篇
  1983年   62篇
  1982年   85篇
  1981年   107篇
  1980年   27篇
  1979年   18篇
  1978年   24篇
  1977年   23篇
  1975年   19篇
  1974年   25篇
  1972年   20篇
  1971年   21篇
  1970年   20篇
  1969年   24篇
排序方式: 共有3497条查询结果,搜索用时 15 毫秒
951.
In investigating the effect of gravitational changes on development, it is instructive to think of altered gravity (delta g) as a teratogen--that is, an environmental factor influencing development. Observed effects on skeletal development include: suppression of morphogenesis in centrifuged mouse limb buds; advanced fusion stages in centrifuged mouse palates; smaller crown rump lengths (CRL) and decreased number of pregnancies in centrifuged rats and mice; altered differentiation of growth plates in young growing rats in space; and decreased length of calcified long bone regions in fetal rats exposed to microgravity in utero. These studies show that delta g is able to alter development in vivo and in vitro and suggest that delta g operates, at least in part, at the cellular level.  相似文献   
952.
A diagnostic tool for detecting damage to spiral bevel gears was developed. Two different monitoring technologies - oil debris analysis and vibration were integrated using data fusion into a health monitoring system for detecting surface fatigue pitting damage on gears. This integrated system showed improved detection and decision-making capabilities as compared to using individual monitoring technologies. This diagnostic tool was evaluated by collecting vibration and oil debris data from fatigue tests performed in the NASA Glenn Spiral Bevel Gear Fatigue Rigs. Data was collected during experiments performed in this test rig when pitting damage occurred. Results show that combining the vibration and oil debris measurement technologies improves the detection of pitting damage on spiral bevel gears.  相似文献   
953.
With the objective of developing Microwave Remote Sensing technology in the country, India has launched a series of Satellites Bhaskara-I and II with the microwave radiometer capability. In this paper, an attempt is made to demonstrate the capability of the brightness temperature data acquired by these radiometers to discriminate various soil moisture conditions of Indian land mass. The analysis show that large areas assessment of soil moisture is possible to a limited extent.  相似文献   
954.
A rocketborne R.F. resonance cone technique is discussed in this paper. The technique is ideal over the geomagnetic equator where the earth's magnetic field is horizontal. The R.F. exciter is fixed along the axis of the rocket. By measuring the semi-angle of this cone various plasma parameters can be derived, such as electron density and temperature.  相似文献   
955.
956.
Extensive studies have been conducted concerning individual mass, temporal and positional distribution of submicron rocky ejecta existing in the satellite-planetary gravitational sphere of influence. The transit time of the major portion of the ejecta that is transported from the satellite's gravitational sphere of influence to the planetary magnetopause is about one week and represents a mass loading pulse occurring each satellite orbit. The mass-flux distributions of lunar ejecta at the surface of the magnetopause for a complete lunar orbit are presented. Spatial mass densities of lunar ejecta in specific zones of the magnetosphere provide a means to compare sporadic interplanetary dust spatial mass densities in the same zones.  相似文献   
957.
The observation of infrared absorption lines by means of a grille spectrometer on board Spacelab 1 allows the determination of Co2 and CO in the low thermosphere and in the middle atmosphere. Equal abundances of CO and CO2 are found at 115 ± 5 km altitude. CO2 is observed to depart from its homospheric volume mixing ratio near 100 km, dropping by a factor of 10,15 km higher. The CO largest number density is observed near 70 km altitude, close to the H Lyman alpha photoproduction peak.The analysis of one run dedicated to the observation of water vapor shows a middle atmospheric mixing ratio of this species within the limits : 3 to 8 ppmv up to 70 km altitude, with the indication of an increase from 30 to 50 km altitude. The H2O mixing ratio drops very rapidly above 70 km.The comparison of the results from strong and weak H2O and CO2 lines shows the need to refine the line profile model.  相似文献   
958.
Differential GPS on board naval vessels has been tested in severe multipath archipelago environment against a commercial laser tracker system. The average location difference was 3 meters and two periodic error components were found. Their tune constants were 1.8 and 28 seconds and the peak amplitudes 1.5 and 1.1 meters, respectively. In addition to these, a clear dependency of location error standard deviation figures on the GPS antenna mounting distance from the sea level was observed.  相似文献   
959.
Air defense systems protect land and maritime resources from air attack. Depending on the regional characteristics and type of conflicting forces, air defense threats vary considerably. In regional conflicts, where forces with similar capabilities are involved and no air-superiority can be achieved, the role of air defense systems becomes critical. In combat terrains containing mountains (in mainland or in small islands), the man-operated or computer-controlled (using passive sensors) anti-aircraft artillery can be highly effective. The simulator presented in this work aims to exploit the capabilities provided by current commercial-off-the-shelf (COTS) communication and multimedia technologies for providing a training environment that improves the personnel capability for effective use of man-controlled anti-aircraft weapons.  相似文献   
960.
A comprehensive programme is proposed for optical observations of Comet Halley based on the wide range of available facilities such as photometers, monochromators, interferometers and a polarimeter. Feasibility study of the proposed investigations with reference to existing facilities is carried out for each technique and needs for additional instrumentation are established. Conclusions for optimum utilization of existing facilities are presented.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号